Selfish DNA is DNA which is prevalent in the genome, not because its phenotypic effect is beneficial, but because it has properties which cause the number of copies of it within the genome to increase with time.

Examples of selfish DNA are:

• Transposons copy themselves to different loci in the genome.
• Microsatellites increase the numbers of concatenated repeats within themselves because of their effect on DNA replication machinery.
• Homing endonuclease genes cleave DNA at its own site on the homologous chromosome, triggering the DNA double-stranded break repair system, which "repairs" the break by copying the HEG onto the homologous chromosome. HEGs have been characterized in yeast, and can only survive by passing between multiple isolated populations or species.
• Meiotic drive enables entire chromosomes to increase their rate of reproduction. In the maize "abnormal 10" (Ab10) chromosome, multiple genes interact to greatly increase the rate at which Ab10 is segregated into the megaspore (egg cell) when heterozygous with a normal chromosome 10.

The paradigm of the selfish gene views all DNA as ultimately being "selfish." However, selfish DNA is distinct from much other DNA in the genome in that the fitness of the latter is intimately tied to the fitness of the organism